Indirectly heated cathodes



March 14, 1961 H. w. COCKRILL INDIRECTLY HEATED CATHODES Filed Dec. 29, 1958 IN VE N TOR Allll I 4 FIG.

Q9?ZZQZ/Aa? 6 5 7 HUSTU/V W, COCKR/LL mom-norm HEATED CATHODES Huston W. Cockrill, Sherborn, Mass., assignor to Raytheon Company, a corporation of Delaware Filed Dec. 29, 1958, Ser. No. 783,265

1 Claim. 01. 313-346) This invention relates generally to cathode constructions and more particularly to cathodes of the indirectly heated type.

Indirectly heated cathodes generally comprise a heater which, either with or without insulating material interposed, is surrounded by a cathode body in the form of a short length of tube, said tube being coated with a substance capable of readily emitting electrons.

It is the primary object of this invention to provide an improved construction in such indirectly heated cathodes.

A more specific object of this invention is to provide an indirectly heated cathode wherein the heater element is rigidly supported within the cathode body.

Another object of this invention is to provide an indirectly heated cathode wherein the heat transfer relationship between the heater element and the emissive surface of the cathode is improved over units of the prior art to reduce the operating temperature differential between the heater and the surface.

A further object of this invention is to provide an indirectly heated cathode wherein a barrier is provided between the emissive surface of the cathode and the heater therefor to prevent the detrimental effects of a reverse migration of emissive material between the surface and the heater.

With the foregoing and other objects in view the invention resides in the following specification and appended claim, certain embodiments and details of construction of which are illustrated in the attached drawing.

The drawing consists of Fig. 1, which is a longitudinal section of an indirectly heated cathode made in accordance with one embodiment of this invention, Fig. 2, which is a longitudinal section taken through an indirectly heated cathode made in accordance with another embodiment of this invention, and Fig. 3, which is a partial longitudinal section similar to Fig. 1, illustrating a modification thereof.

Referring more particularly to the drawing, the cathode, generally indicatcd at 1, includes an outer sleeve 2 of molybdenum or other suitable material. The sleeve 2 may assume various cross-sectional shapes such as ovoid, circular or substantially flat, such shape not being considered as pertinent to the concepts of this invention. Within the confines of the sleeve 2 adjacent to one end thereof is placed an impregnated sintered pellet 3 of tungsten or other suitable material which pellet constitutes the emitting surface for the cathode 1.

A wire heater 4 of tungsten or other material is placed in wound form within substantially the entire remaining portion of the sleeve 2 not occupied by the pellet 3, with the spacing between turns of the heater 4 and between the heater and the sleeve being substantially uniform to aid in even heat transfer within the cathode 1. The heater 4 is preferably coated with an insulating material 5 such as aluminum oxide. After the heater 4 is inserted within sleeve 2 the surrounding space is filled with molybdenum 2 ,975,322 Patented Mar. 14, 1961 powder 6 or other suitable metal powder such as tungsten. It has been found advantageous to use a vehicle for the molybdenum powder such as a nitrocellulose solution. The entire assembly is then heated to the sintering temperature for molybdenum in a reducing atmosphere. After sintering the molybdenum, the cathode sleeve 2 and the encased heater 4 comprise a rugged unit in which detrimental vibration of the heater 4 is eliminated. With this rigid construction it becomes possible to work with the terminals 7 and 8 of heater 4 without affecting the position of the heater within sleeve 2.

In order to reduce or eliminate the detrimental effect of emissive metal from the pellet- 3 on the heater insulating material 5, a barrier 9 illustrated as a disk of molybdenum, tungsten or other suitable material, is positioned within sleeve 2 between the pellet 3 and the sintered molybdenum 6. This barrier substantially eliminates reverse migration of emissive material from the pellet 3 toward the heater 4. Permeation of the insulating material about the heater 4 by the emissive metal would result in a lowered electrical resistivity and would require higher heater current to maintain a desired cathode operating temperature. In lieu of the disk barrier 9, the rear of the cathode pellet 3 may be peened or burnished on the surface thereof facing the heater 4, as illustrated in Fig. 3, to create an integral or built-in barrier 10 which is effective in retarding migration of electrons from the pellet 3 rearwardly toward the heater 4. The peening or burnishing of the rear of the cathode material closes the pores thereof to form an imperforate surface and effectively results in a barrier it which, if not superior, is at least equal to the provision of a barrier comprised of one or more disk barriers. It also results in improved heat transfer characteristics and allows the use of simpler construction techniques.

The construction of the cathode 1 as fully described above results in a unit wherein extended heater life is obtained over similar constructions of the prior art due to the lower operating temperatures at which the heater 4 may be operated for a given or desired cathode operating temperature. This is made possible by the uninterrupted thermally conductive path from the heater 4 to the pellet 3 through the sintered molybdenum 6, the barriers 9 or 10 and along the sleeve 2 to the emissive pellet 3. By avoiding the necessity of much higher heater temperatures to obtain a particular cathode operating temperature, the life of the heater is greatly extended. Also by the use of the barriers 9 or 10 the insulating material 5 about the heater 4 is protected against detrimental infiltration of migratory emissive material without decreasing the efficiency of the thermal conductive path between the heater and the pellet, to further act as an aid toward extending heater life.

Fig. 2 discloses a cathode Where the outer sleeve 12, corresponding to sleeve 2 of Fig. 1, has been impregnated with tungsten. The heater '14 shown in Fig. 2, having leads 17 and 18 and an insulating coating 15 is a straightthrough heater, and acts in a manner similar to that described for Fig. 1. The emissive material of the pellet 3 in Fig. 1 may be applied as a sintered sleeve if the sleeve 12 is not impregnated with the emissive material as shown in Fig. 2. The barrier 19 which is between the sleeve and the heater acts in a manner similar to the barrier 9 of Fig. l to retard the migration of the emissive material toward the heater 14 through the sintered metal powder 16.

It will therefore be apparent to one skilled in the art that the invention provides a novel improvement in cathode construction making it possible to operate at much smaller temperature differentials between the heater and the cathode itself to insure longer heater life. The structure in addition to its heat transfer features includes the emissive material barrier and the rigid enclosure for the heaters 4 and 14 to prevent vibration as well as heat damage to the heater. It is realized that various structural and material modifications of the construction illustrated might be devised by one skilled in the art,- which would be considered well within the purview of this inventor and which should be considered as being Within the scope of the appended claim.

I claim:

, An indirectly heated cathode comprising a metal sleeve, an impregnated sintered metal pellet, constituting the emissive surface for the cathode, supported within the sleeve at one end thereof and having an imperforate surface forming an integral part of said pellet disposed metal and said imperforate surface tightly contacting each other so as to constitute a virtually uninterrupted thermal transfer path between the heater and the pellet and said encasing metal and said sleeve tightly contacting each other so as to constitute another virtually uninterrupted thermal transfer path between the heater and the pellet, and said imperforate surface constituting a barrier against migratory emissive material flow from the pellet to the heater.

References Cited in the file of this patent UNITED STATES PATENTS 2,724,788 Edwards Nov. 22, 1955 2,798,182 Costa July 2, 1957 2,813,220 Coppola Nov. 12, 1957 FOREIGN PATENTS 163,032 I Australia May 25, 1955 701,871 Great Britain Jan.'6, 1954 

